1. Field of the Invention
The present invention relates to a blind rivet nut setting device having a drawing mandrel, a motor, a first selectively interruptible drive line disposed between the motor and the drawing mandrel and a second drive line disposed between the motor and the drawing mandrel. The first drive line, when not interrupted, rotates the drawing mandrel. The second drive line has a drive element and an output element, which is connected to the drawing mandrel. The drive element is axially movable along a path which has an idle stroke portion and an effective stroke portion.
2. Discussion of the Related Art
Blind rivet nuts typically have a rivet shaft that has an internal thread and a set head. The rivet shaft is guided in a hole of a workpiece which is to be fastened to the blind rivet nut. While the set head is in contact with the material of the workpiece, an axial tension is applied to the rivet shaft by means of a drawing mandrel which is screwed into the internal thread of the rivet shaft. The tension causes a compression of the rivet shaft and a crowning of the rivet shaft material, which forms what is known in the art as a snap head. The blind rivet nut is now fastened to the material in the form of a blind rivet. Of course, several layers of material can be connected together with a blind rivet nut. After unscrewing the drawing mandrel, the thread in the workpiece remains intact and is available for other uses.
Because the drawing mandrel must at first be screwed into the blind rivet nut and then unscrewed alter the setting, the work process for setting a blind rivet nut consists essentially of three steps: (1) Placing the blind rivet nut on the drawing mandrel; (2) Setting; and (3) Resetting. The placing of the nut on the mandrel must be finished before the setting step and requires a predetermined minimum amount of torque. If the blind rivet nut contacts the mouthpiece of the blind rivet nut setting device after the nut has been screwed in place, a premature compression of the rivet shaft may occur. The setting process must also be controlled fairly accurately. If the setting stroke or effective stroke is too small, the blind rivet nut will not be fastened to the workpiece with the required strength. If, however, the setting stroke is too large, the blind rivet nut could be damaged, thereby drastically reducing the reliability of the connection. Finally, the resetting process must take place so that the drawing mandrel can be unscrewed from the blind rivet nut with a predetermined amount of torque.
In EP 0 119 007 A2, a blind rivet nut setting device has a first drive line that includes a ratchet type coupling. When the ratchet coupling reaches a predetermined torque the coupling permits slippage in one direction. A drawing mandrel can be rotated and screwed into the blind rivet nut. After the blind rivet nut contacts the mouthpiece, the coupling will slip. The drawing mandrel can be unscrewed from the blind rivet nut by rotating the drawing mandrel with full torque in the reverse direction. A threaded spindle is connected to a motor. The threaded spindle has a slide which is axially displaced during a rotation of the threaded spindle. After an idle stroke, the slide contacts the stop of a spacer. This spacer then exerts tension on the drawing mandrel. The entire spacer is moved by the slide until the spacer reaches another stop. This other stop can be adjusted by insertion of additional spacers, which thereby determines the distance of the effective stroke.
Another blind rivet nut setting device is known from DE 19 45 820 U 1, which includes a drive motor that rotates in only one direction. A direction reversing drive is used to control the movement of the mandrel. When the blind rivet nut is set, the device must be compressed axially so that the necessary parts can come into engagement with each other. A slip coupling is provided in this device so that when the drawing mandrel has been threaded far enough onto the blind rivet nut, the coupling begins to slip. A torque limiting device is also provided which interrupts the drive of the drawing spindle when the power required for forming the snap head has reached a predetermined magnitude.
In both of these devices, it is relatively difficult to assure that the quality of the fastening of the blind rivet nut will remain consistent throughout the various sizes of blind rivet nuts that can be set with each device. In the '007 reference, a change of the spacer is required to vary the effective stroke. However, because this operation requires disassembling of the blind rivet nut setting device this operation can not be done in the field with the necessary care. In the '820 reference, one is dependent on the quality of the torque limiting device. However, it is known from experience, that the torque limit will change over time, e.g. , through contamination. Therefore, the reliability of the connection of the blind rivet nut with the workpiece material will suffer.